Hybrid bipolar junction transistor

ABSTRACT

Bipolar junction transistors including inorganic channels and organic emitter junctions are used in some applications for forming high resolution active matrix displays. Arrays of such bipolar junction transistors are electrically connected to thin film switching transistors and provide high drive currents for passive devices such as organic light emitting diodes.

FIELD

The present disclosure relates to thin-film electronic device structuresand technology and, more particularly, to bipolar junction transistors,active matrix structures such as backplanes incorporating suchtransistors, and the fabrication of bipolar junction transistors andactive matrix structures using both inorganic and organic materials.

BACKGROUND

Bipolar junction transistors include emitter, collector and baseregions. A biasing voltage applied between a base contact and an emittercontact allows control of collector current. Bipolar junctiontransistors can be employed as switches or amplifiers. The operation ofheterojunction bipolar transistors, which are a type of bipolar junctiontransistors, is based on a bandgap difference between the emitter andbase. Significant current gains can be obtained through the use ofheterojunction bipolar transistors.

Active matrix devices such as displays (e.g. televisions, laptopmonitors), imagers (e.g. x-ray imagers) and sensors typically usehydrogenated amorphous silicon (a-Si:H) and, in some applications,low-temperature poly-silicon (LTPS) thin-film transistor (TFT)backplanes on glass or, for flexible devices, clear plastic. However,for very high resolution applications (>1000 pixels per inch (ppi)),such as micro-displays or pico-projectors, the carrier mobility ofa-Si:H (electron mobility of about 1 cm²Ns) is too low to providesufficient drive current at short TFT channel widths. For applicationsrequiring high drive current, such as active matrix organic lightemitting diode (AMOLED) displays, it is necessary to shrink the gatelength and/or increase the gate width of a-Si:H transistors. This leadsto increasing the processing cost of a-Si:H active matrix circuits dueto the relatively small gate lengths as well as a significant trade-offin display resolution due to larger gate widths. LTPS is more expensivethan a-Si:H, but capable of providing higher drive currents. Thedevice-to-device variation of threshold voltage and mobility in LTPStransistors requires compensation circuitry that limits the resolutionof the active matrix. Single crystalline silicon (c-Si) has been used asan alternative for very high resolution backplanes, but processing c-Sican require high temperatures not compatible with glass substratescurrently used in manufacturing a-Si:H or LTPS devices or clear plasticsubstrates that may be used.

BRIEF SUMMARY

In accordance with the principles discussed herein, hybrid bipolarjunction transistors and active matrix structures including suchtransistors are provided.

A bipolar junction transistor is disclosed that includes a dopedinorganic semiconductor layer, an emitter electrode, a base electrodeand a collector electrode. A first structure is operatively associatedwith the emitter electrode for transporting charge carriers having afirst charge type into the inorganic semiconductor layer and suppressingdiffusion of charge carriers having a second charge type opposite fromthe first charge type from the inorganic semiconductor layer. The firststructure is positioned between the emitter electrode and the inorganicsemiconductor layer and includes a first organic semiconductor carriertransport layer for transporting the charge carriers having the firsttype. A second structure is operatively associated with the baseelectrode for transporting charge carriers having the second charge typeinto the inorganic semiconductor layer and suppressing diffusion ofcharge carriers having the first charge type from the inorganicsemiconductor layer. The second structure is positioned between the baseelectrode and the inorganic semiconductor layer. A third structure isoperatively associated with the collector electrode for transportingcharge carriers having the first charge type into the inorganicsemiconductor layer and suppressing diffusion of charge carriers havingthe second charge type from the inorganic semiconductor layer. The thirdstructure is positioned between the collector electrode and theinorganic semiconductor layer.

An exemplary method includes obtaining a bipolar junction transistorincluding a doped crystalline inorganic semiconductor layer and emitter,base and collector contact structures operatively associated with thecrystalline inorganic semiconductor layer. The emitter contact structureis configured for transporting charge carriers having a first chargetype into the inorganic semiconductor layer and suppressing diffusion ofcharge carriers having a second charge type opposite from the firstcharge type from the inorganic semiconductor layer. The emitter contactstructure includes a first organic semiconductor carrier transport layerfor transporting the charge carriers having the first type. The methodfurther includes causing diffusion of charge carriers having the firstcharge type into the inorganic semiconductor layer from the emittercontact structure and, within the emitter contact structure, suppressingdiffusion of charge carriers having the second charge type opposite fromthe inorganic semiconductor layer. The bipolar junction transistor iscaused to provide electrical current to an electronic device.

An exemplary structure includes an inorganic semiconductor layeradjoining an electrically insulating layer and an array of bipolarjunction transistors, each of the bipolar junction transistors includingan emitter contact structure operatively associated with the inorganicsemiconductor layer. Each emitter contact structure is configured fordiffusing a first type of charge carrier into the inorganicsemiconductor layer and suppressing the diffusion of a second type ofcharge carrier opposite from the first type of charge carrier from theinorganic semiconductor layer. Each emitter contact structure includesan organic semiconductor carrier transport layer for facilitatingtransport of the charge carriers having the first type. The structurefurther includes an array of thin film switching transistors, each ofthe thin film switching transistors being electrically connected to oneof the bipolar junction transistors, and an array of electronic devices,each of the electronic devices being electrically connected to one ofthe bipolar junction transistors.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

Substantial beneficial technical effects are provided by the exemplarystructures and methods disclosed herein. For example, one or moreembodiments may provide one or more of the following advantages:

-   -   Enabling higher resolution displays due to relatively high drive        current and/or low operation voltage compared to a-Si:H or        organic TFTs;    -   Same fabrication infrastructure for backplane and frontplane        structures feasible;    -   Low-temperature processing compatible with flexible and low-cost        substrates;    -   Larger minimum device feature size (channel length) compared to        a-Si:H devices allowing lower lithography costs.    -   Lower power consumption compared to a-Si:H or organic TFT        backplanes due to lower operation voltages.

These and other features and advantages of the disclosed methods andstructures will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of active matrix pixel circuits ina TFT/OLED display;

FIG. 2A shows a schematic illustration of an emitter region for a hybridbipolar junction transistor;

FIG. 2B shows a schematic illustration of an alternative embodiment ofan emitter region for a hybrid bipolar junction transistor;

FIG. 3 shows an energy band diagram for an exemplary emitter region of ahybrid bipolar junction transistor;

FIG. 4 shows a schematic illustration of an exemplary hybrid bipolarjunction transistor;

FIG. 5 shows a schematic illustration of an alternative exemplary hybridbipolar junction transistor;

FIG. 6A shows a schematic illustration of an exemplary test structureincluding emitter and collection electrodes formed on an organic layer;

FIG. 6B is a greatly enlarged schematic illustration of a portion of theexemplary test structure shown in FIG. 6A;

FIG. 7 is a graph showing collector and base current as a function ofemitter-base voltage;

FIG. 8 is a schematic illustration of an exemplary backplane including ahybrid bipolar junction transistor functioning as a drive transistor,and

FIG. 9 is a schematic illustration of an alternative embodiment of anexemplary backplane including a hybrid bipolar junction transistorfunctioning as a drive transistor.

DETAILED DESCRIPTION

Thin-film hybrid bipolar junction transistors are disclosed that includecrystalline, inorganic channels and organic emitter junctions. Suchtransistors are used in some embodiments for forming high resolutionactive matrix displays. A backplane refers to an array of transistors(active devices) used for addressing and programming passive devicessuch as light emitting diodes, liquid crystal displays, photosensitivematerials (e.g. for x-ray imaging), or sensors (e.g. piezoelectricmaterials for sensing pressure). The backplane also contains addresslines, program lines, power supply lines, and typically storagecapacitors which are fabricated using the same process technology asthat of the transistors. Arrays of passive devices addressed/programmedby the backplane are typically referred to as the frontplane. An activematrix refers to the combination of a backplane and a frontplane.Schematic pixel circuits of active-matrix arrays comprised of OLEDs areillustrated in FIG. 1. An exemplary OLED includes one or more layers oforganic electroluminescent material disposed between two electrodes. Thecircuit 20 illustrated in FIG. 1 is a 3×2 active matrix, i.e. comprisedof six (6) pixels. A switching thin film transistor (TFT) 22, a storagecapacitor 24, and a driver thin film transistor 26 are operativelyassociated with an OLED 28. One TFT 22 is employed to start and stopcharging of a storage capacitor while the other 26 functions as acurrent source to provide a constant current for each pixel. The storagecapacitor maintains a constant voltage on a charged pixel betweenrefresh cycles. The frontplane of passive elements (OLEDs) isintegrated, e.g. laminated, onto a backplane including an array of TFTelements to control current flowing to the passive elements. The selectand data lines respectively transfer the switching andprogramming/readout signals. As discussed below, hybrid driver thin filmtransistors having crystalline channels and organic emitter junctionsare disclosed that provide a high and stable drive current for passivedevices such as OLEDs thus allowing high resolution and low powerconsumption.

High resolution active matrix structures are fabricated using techniquesdescribed below. A backplane layer including active semiconductordevices is formed in some embodiments using a semiconductor-on-insulatorsubstrate. The semiconductor-on-insulator substrate may be prepared byvarious layer transfer techniques known in the art such as controlledspalling, epitaxial layer lift-off or SMART CUT®. In these techniques, athin layer of crystalline semiconductor is transferred from a hostsubstrate and bonded onto an insulating handle (carrier) substrate. Thetransfer and/or bonding methods are different in different techniques.Driver transistors as disclosed herein are formed using thesemiconductor layer of the substrate along with additional circuitelements that provide other functions such as computing or sensing. Inother embodiments, the starting substrate is fabricated bycrystallization of non-crystalline materials grown on an insulatingsubstrate using known techniques such as laser crystallization.Substantially higher drive currents and/or lower operation voltages maybe obtained compared to a-Si:H or organic TFTs due to the highermobility of crystalline channels in the driver transistors. As usedherein, the term “crystalline” refers to single-crystalline(mono-crystalline), or poly-crystalline (multi-crystalline); the term“non-crystalline” refers to amorphous, nano-crystalline andmicro-crystalline. Although the use of crystalline channel materialsprovide substantial functional advantages over prior art structures, itwill be appreciated by those skilled in the art that the principles ofthis disclosure are also applicable to non-crystalline channelmaterials.

FIG. 2A shows an exemplary schematic emitter contact structure 30 for ahybrid bipolar junction transistor on a doped crystalline siliconsubstrate layer 32. The substrate is n-type in this exemplaryembodiment. As used herein, “n-type” refers to the addition ofimpurities that contribute free electrons to an intrinsic semiconductor.In a silicon containing substrate, examples of n-type dopants, i.e.impurities, include but are not limited to antimony, arsenic andphosphorous. The doped semiconductor layer 32 can be formed as acontinuous layer as shown during fabrication of asemiconductor-on-insulator (SOI) wafer. Ion implantation canalternatively be employed following SOI substrate wafer fabrication toform the doped layer. The layer 32 is between 20 nm-1 μm in thickness inone or more embodiments. The doping concentration of the layer 32 isbetween 10¹⁵ to 10¹⁹ cm⁻³ in one or more embodiments. The emitterstructure 30 of the exemplary device includes an emitter electrode 34, ahole transport layer 36, an electron blocking layer 38 and a passivationlayer 40. The emitter electrode is formed from a preferably highworkfunction material such as gold, platinum, palladium or nickel. Thehole transport layer 36 is a layer of organic semiconductor material.The electron blocking layer 38 and passivation layer 40, if employed,are preferably organic. Organic materials may be evaporated at or closeto room-temperature or grown from a solution thus allowing lower growthcosts compared to typical inorganic materials which require more complexgrowth techniques or more expensive precursors. In some embodiments,some or all of the blocking, passivation and transport functions areserved by a single layer. The order of the transport layer 36 and theblocking layer 38 are reversed in some embodiments. It will beappreciated that the electron blocking layer may impede carriertransport due to band offset with respect to the transport layer 36 orinherent material property (effective mass). Examples of organicmaterials which can provide hole transport and/or electron blockingfunctions include but are not limited to pentacene, rubrene, anthracene,poly(3-hexylthiophene) (P3HT); tetraceno[2,3-b]thiophene;α-sexithiophene; poly(3,3′″-didodecylquaterthiophene);poly(2,5-bis(3-decylthiophen-2-yl)thieno[3,2-b]thiophene);N,N′-Bis(3-methylphenyl)-N,N′-diphenyl-benzidine (TPD);N,N′-Bis(phenanthren-9-yl)-N,N′-bis(phenyl)-benzidine (PAPB);4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP); 1,3-Bis(N-carbazolyl)benzene(mCp); 4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine](TAPC);2,2′-Dimethyl-N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine(α-NPD);9,9-Dimethyl-N,N′-di(1-naphthyl)-N,N′-diphenyl-9H-fluorene-2,7-diamine(NPB); N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine(NPD); N,N′-Di(2-naphthyl-N,N′-diphenyl)-1,1′-biphenyl-4,4′-diamine(β-NPB); Tri-p-tolylamine;4,4′,4″-Tris[phenyl(m-tolyl)amino]triphenylamine;Tris(4-carbazoyl-9-ylphenyl)amine (TCTA); Tetra-N-phenylbenzidine (TPB);1,3-Bis(triphenylsilyl)benzene; poly-aniline;poly(3,4-ethylenedioxythiophene);poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS);poly(3,4-ethylenedioxythiophene); tetracyanoethylene;poly(thiophene-3-[2-(2-methoxyethoxyl)ethoxy]-2,5-diyl);bis-poly(ethyleneglycol) (PEDOT:PEG); 7,7,8,8-Tetracyanoquinodimethane;and combinations thereof.

FIG. 2B shows an exemplary schematic emitter contact structure 50 for analternative embodiment of a hybrid bipolar junction transistor on adoped crystalline silicon substrate layer 52. The substrate is p-type inthis exemplary embodiment. In a silicon-containing substrate, examplesof p-type dopants include but are not limited to boron, aluminum,gallium and indium. The doped layer 52 can be formed as a continuouslayer as shown during fabrication of a semiconductor-on-insulator (SOI)wafer. Ion implantation can alternatively be employed following SOIsubstrate wafer fabrication to form the doped layer. The emitter contactstructure 50 of the exemplary device includes an emitter electrode 54,an electron transport layer 56, a hole blocking layer 58 and apassivation layer 60. The emitter electrode is preferably formed from alow workfunction material such as erbium or magnesium. The electrontransport layer 56 is a layer of organic semiconductor material. Thehole blocking layer 58 and passivation layer 60, if employed, arepreferably organic. In some embodiments, some or all of the blocking,passivation and transport functions are served by a single layer. Theorder of the transport layer 56 and the blocking layer 58 are reversedin some embodiments. Examples of organic materials which can provideelectron transport and/or hole blocking functions include but are notlimited to bathocuproine (BCP); bathophenanthroline (BPhen);3-(Biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole(TAZ); 2-(4-Biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD);bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum;2,5-Bis(1-naphthyl)-1,3,4-oxadiazole (BND);2-(4-tert-Butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (Butyl-PBD);Tris-(8-hydroxyquinoline) aluminum (Alq3); hexadecafluoro copperphthalocyanine (F₁₆CuPc); naphthalene diimide derivatives; perylenediimide derivatives; C₆₀; and combinations thereof.

FIG. 3 is an energy band diagram relating to a specific emitterstructure such as that shown in FIG. 2A, though without a discreteelectron blocking layer 38. In this exemplary embodiment, a highworkfunction metal, namely gold, functions as a hole generator,generating holes in a pentacene layer that serves as the hole transportlayer. Pentacene is an organic semiconductor material that loseselectrons to gold, generating holes in the pentacene. The pentacene holetransport layer injects holes through the passivation layer into then-type silicon layer primarily by diffusion. Due to its low electronaffinity, pentacene also helps block electron diffusion from the n-typesilicon layer towards the gold electrode. The passivation layer, forexample a 9,10-phenanthrenequinone (PQ) layer, also helps block electrondiffusion towards the gold electrode while serving its primaryelectrical passivation function with respect to the surface of thesilicon layer. Lowest unoccupied molecular orbital (LUMO) and highestoccupied molecular orbital levels are indicated for the pentacene andpassivation (PQ) layers.

Exemplary hybrid bipolar junction transistor devices are shown in FIGS.4 and 5. The same reference numerals are employed in the two figures todesignate similar elements. The device 100 shown in FIG. 4 is formedusing a substrate that includes an n-type crystalline semiconductorlayer 102 that includes silicon. In some embodiments, the semiconductorlayer 102 is a pure crystalline silicon layer doped with an n-typedopant. In other embodiments, the substrate is comprised of other groupIV materials such as Ge, SiGe, SiC, SiGeC or GeC. In other embodiments,the substrate is comprised of III-V or II-VI compound semiconductors.The semiconductor layer directly contacts an electrically insulatinglayer 104. In one or more embodiments, the electrically insulating layeris a buried oxide (BOX) layer. The insulating layer 104 in an exemplaryembodiment is between 5-200 nm, but may also be thicker or thinner forsome applications. The electrically insulating layer 104 in oneexemplary embodiment is a high quality silicon dioxide layer that isthermally grown, though other buried insulators such as boron nitride(BN) and aluminum oxide (Al₂O₃) may alternatively be employed in someembodiments. High quality buried oxides are generally characterized byrelatively low interface trap densities (D_(it)). The exemplary device100 includes an emitter structure 30 as shown in FIG. 2A. As discussedabove, the emitter structure includes an emitter electrode 34, a holetransport layer 36 and an electron blocking layer 38. The collectorstructure 70 includes a collector electrode 74, a hole transport layer76, and an electron blocking layer 78. The collector structure 70 andthe emitter structure 30 have the same elements in some embodiments,which simplifies the fabrication process of the device 100. In otherembodiments, the collector structure and emitter structure are formed atleast in part from different materials. The base structure 80 of thedevice 100 includes a base electrode 84 such as a layer of lowworkfunction material, an electron transport layer 86, and a holeblocking layer 88. In some embodiments, the electron transport layer isan organic semiconductor layer. In preferred embodiments, all thetransport, blocking and passivation layers are comprised of organicmaterials in order to take full advantage of the benefits of organicmaterials including the low cost and low-temperature processingcapability. The thickness of the transport and blocking layers istypically in the range of 1-100 nm with the thickness range 5-50 nmbeing employed in one or more embodiments; however, it should beunderstood that thicker or thinner layers may be employed as well. Thethickness of the passivation layer is typically in the range of 1-25 nm,although thinner or thicker layers may be employed as well. Thethickness of the electrodes is typically in the range of 10-50 nm,although thinner or thicker layers may be employed as well.

In some embodiments, the silicon-based substrate layer is recessed usingknown techniques such as a selective etch process. A device 110 asschematically illustrated in FIG. 5 includes an n-type, silicon-basedsemiconductor layer 112 adjoining a buried oxide (BOX) layer 114. Apassivation layer 116 directly contacts the semiconductor layer 112 aswell as the buried oxide layer. The passivation layer 116 is preferablyorganic. Exemplary passivation materials include poly(3-hexylthiophene)(P3HT) and 9,10-phenanthrenequinone (PQ). Other passivation materialsare known in the art and could possibly be employed. The passivationlayer saturates the dangling bonds at the surface of the crystalline Sisubstrate and therefore reduces minority carrier recombination at thesurface of the crystalline Si substrate. In some embodiments, thepassivation layer is comprised of an organic monolayer formed byimmersing the substrate is a solution of a long-chain alcohol or thiol.The device 110 includes a base structure 80 as described above withrespect to the device 100. The emitter structure 120 and the collectorstructure 130 of the device 110 are formed on the passivation layer 116.While the base structure is formed entirely over the semiconductor layer112, the emitter and collector structures 120, 130 are formed onlypartially over the semiconductor layer 112. The emitter structureincludes an emitter electrode layer 124, a hole transport layer 126 andan electron blocking layer 128. As discussed above, the collectorstructure may include layers 134, 136, 138 that are the same as thosepresent as the emitter structure to facilitate device fabrication. Thebase current may be reduced through the use of a recessed semiconductorlayer, thereby increasing the current gain of the bipolar transistordevice. It will be appreciated that devices similar to those shown inFIGS. 4 and 5 can be formed using emitter structures having electrontransport and hole blocking layers substantially as shown in FIG. 2B. Insuch cases, the semiconductor layer would be p-type and the basestructure would include hole transport and electron blocking layers. Theexemplary devices 100, 110 provide an overall device function (e.g.electrical characteristics in a circuit) which is, in principle, thesame as that of a conventional bipolar transistor. In that sense, theemitter portion of the exemplary devices 100, 110 is analogous to theemitter portion of a conventional p-n-p transistor which is comprised ofa p+/n junction; the base portion of each exemplary device 100, 110 isanalogous to the base portion of a conventional p-n-p transistor whichis comprised of a n+/n junction, the collector portion of each exemplarydevice 100, 110 is analogous to the collector portion of a conventionalp-n-p transistor which is comprised of a p+/n junction. The electricaloperation of the devices 100, 110 as a circuit element is the same asthat of a conventional p-n-p transistor. Similarly, if a p-typecrystalline substrate is used, the disclosed devices are analogous toconventional n-p-n transistors.

A simplified test device 150 is schematically illustrated in FIG. 6. Thetest device includes a crystalline silicon substrate including a heavilydoped p+ layer 152 and a lightly doped p-type epitaxial layer 154. Thelayers 152, 154 have thicknesses of 700 μm and 2.5 μm, respectively. Anorganic monomer passivation layer 156 is formed on the top surface ofthe epitaxial layer 154 and a pentacene layer 158 is formed on thepassivation layer. The organic monomer passivation is formed by firstetching the native oxide of silicon in dilute hydrofluoric acid andimmediately immersing the substrate in 1-dodecanol (CH₃—(CH₂)₁₁—OH)followed by heating at elevated temperatures (90-150° C.). Other longchain alcohols or thiols of the form CH₃—(CH₂)_(n)—XH where element Xrefers to oxygen (O) or sulfur (S) and n is in the range of 4-21 may bealso used. In this process, Si—H bonds of the etched silicon surfacereacts with alcohol or thiol functionality and forms covalently bondedether or thioether on the surface which passivates the surface, asillustrated in FIG. 6B. In this exemplary embodiment, the pentacenelayer 158 has a thickness of approximately 50 nanometers (nm) and isformed by thermal evaporation at room temperature with an averageevaporation rate of approximately 0.3 angstroms per second (Å/sec),using a solid (powder) evaporation source material having a purity ofhigher than 99%. (In other embodiments, the pentacene layer 158 may bealternatively formed using a solution containing a pentacene precursorsuch as 13,6-N-Sulfinylacetamidopentacene, and applied by spin-coatingfollowed by curing. A solvent such as chloroform, chlorobenzene orcombinations thereof may be used to prepare the solution. Curing may beperformed at temperatures close to 200° C. in a dry atmosphere, althoughlower temperatures may be used as well.) An aluminum contact layer 160directly contacts the substrate layer 152 and functions as a baseelectrode. Two aluminum contacts 162, 164 are formed on the pentacenelayer, one functioning as an emitter electrode and the other as acollector electrode. The aluminum contacts 162, 164 were deposited bythermal evaporation at room temperature through a shadow mask, at anaverage evaporation rate of 5 Å/sec. As known in the art, a thin layer(typically 1-3 nm) of lithium fluoride (LiF) or other materials (such atransition metal oxides) may be disposed between the pentacene layer 158and the aluminum contacts 168 and 164 (typically by thermal evaporation)to avoid chemical reaction between aluminum (Al) and pentacene andtherefore improve the long-term chemical stability of the device. Thisstep has been omitted in this exemplary embodiment for simplicity. (Asknown in the art, the LiF layer may also serve to improve electroninjection from aluminum into pentacene by lowering the effectiveworkfunction of the Al/LiF contact.) The base current of the test deviceis high resulting in a current gain of about two (2). This is because oflarge (1.5 mm²) contact pads and large (25 μm) emitter-collectorseparation. Higher gain is expected with smaller contact pads andsmaller collector-emitter separation. Bipolar function is clearlyobserved in the test device. FIG. 7 shows collector and base current ofthe test device 150 as a function of emitter-base voltage.

An exemplary vertically integrated backplane 200 including hybridbipolar junction transistors 202 is schematically shown in FIG. 8. Inthis exemplary embodiment, an organic thin film transistor 204, whichfunctions as a switch, is integrated on top of each hybrid bipolarjunction transistor. The hybrid bipolar junction transistors function asdrive transistors for passive devices, such as OLEDs shown in FIG. 1.The active devices are formed on an insulating substrate 206. A thincrystalline silicon layer 208 adjoins the substrate 206. As discussedabove with respect to FIGS. 2A and 2B, the silicon layer 208 can ben-type or p-type. Organic stacks 210, 212, 214 are formed on the siliconlayer 208. The organic stacks correspond to passivation, hole transport,electron blocking, electron transport, and hole blocking layers such asthose described above with respect to FIGS. 4 and/or 5. The metal layer216 (M1) includes the emitter, base and collector electrodes of thehybrid bipolar junction transistor 202. Each hybrid bipolar junctiontransistor is electrically connected to an indium tin oxide (ITO)electrode 218. A first passivation layer 220 is formed on the siliconlayer 208. The organic thin film transistors 204 in this exemplaryembodiment are inverted-staggered bottom-gate devices. They includechannels comprised of organic layers 222 and dielectric layers 224formed between the channel layers and a metal layer 225. The transistors204 are electrically connected to the drive transistors 202 by a metallayer 226 (M3). The drain of each transistor 204 is electricallyconnected to the base electrode of one of the hybrid bipolar junctiontransistors 202 in the exemplary embodiment as shown. The transistors202, 204 are incorporated in a circuit as shown in FIG. 1 in someembodiments. A third passivation layer 228 is formed over thin film(switching) transistors 204. The passivation layers 209, 220, 221 and228 are comprised of insulating materials which may or may not beorganic. Inorganic passivation layers are preferably grown by thermalevaporation, e-beam evaporation or atomic layer deposition to avoid theuse of plasma which may damage organic materials; however the growthmethods involving plasma such as PECVD or sputtering may be used in someembodiments. Examples of inorganic insulators include but are notlimited to silicon dioxide, silicon nitride, aluminum oxide, hafniumoxide or combinations thereof. Organic passivation layers are typicallygrown by thermal evaporation or spun from a solution. Examples oforganic insulating materials that can be employed for one or more of thepassivation layers include but are not limited to parylene, polyimide,polystyrene and polyvinyl alcohol (PVA). In some embodiments, theorganic insulating materials may include a photo-sensitizing material tofacilitate the patterning of these materials using photolithography. Inone example, ammonium dichromate is used as a photo-sensitizer in a PVAsolution and applied by spin coating. In some embodiments thepassivation layer 228 may also serve as an edge planarization layer toavoid shorts between the OLED cathode (not shown) and the OLED anode,layer 218 (e.g. ITO). In some embodiments edge planarization may befacilitated by a reflow process which involves low temperature annealingof a polymeric passivation layer after spin-coating and patterning thepolymeric passivation layer. Amorphous silicon is not required in theexemplary backplane. The fill factor of each pixel is improved byreducing the active device area, thus providing more area for the OLED(not shown) operatively associated with each switching and drivetransistor. It will be appreciated that, in alternative embodiments ofthe backplane, the organic thin film transistors 204 may be incorporatedon the side rather than on top of the electrically associated drivetransistors 202.

A second exemplary vertically integrated backplane 300 is shown in FIG.9. The backplane 300 includes elements that are also found in thebackplane 200 discussed above and are designated with the same referencenumerals. In this exemplary embodiment, a top-gate staggered organicthin film transistor 304 is electrically connected to a thin-film drivetransistor 202. A metal layer 306 (M3) adjoins a gate dielectric layer308. Materials such as silicon dioxide or high-k materials such as HfO₂or Al₂O₃ can be employed as gate dielectric layers. Organic dielectriclayers such as parylene and polyimide are used in some embodiments. Anorganic channel layer 310 adjoins the gate dielectric layer. Each thinfilm switching transistor 304 is electrically connected to the baseelectrode of a drive transistor 202 by a second metal layer 312.

A silicon-on-insulator (SOI) wafer comprised of a thin crystallinesemiconductor layer on a buried oxide (BOX) insulator, which is in turnon a bulk silicon (handle) substrate is employed in some embodiments asa starting substrate for fabricating hybrid bipolar junctiontransistors, such as the transistors described above. The hybridtransistors are characterized by their inclusion of both asilicon-containing and organic semiconductor elements as electricallyactive parts of the devices. The thickness of the semiconductor layer isbetween 20 nm-1 μm in some exemplary embodiments although thinner orthicker layers may be used as well. Relatively thin semiconductor layersfacilitate the production of mechanically flexible active matrixstructures. Exemplary single crystal materials for the crystallinesemiconductor layer include silicon and silicon-containing films such assilicon germanium. Compound III-V and II-VI semiconductors may also beused. The insulator layer 34 in an exemplary embodiment is between 5-200nm, but may also be thicker or thinner for some applications.

Given the discussion thus far and with reference to the exemplaryembodiments discussed above and the drawings, it will be appreciatedthat, in general terms, an exemplary bipolar junction transistor isprovided that includes a doped, inorganic semiconductor layer 102 or112, an emitter electrode 34 or 124, a base electrode 84, and acollector electrode 74 or 134, as shown in the exemplary devicesdepicted in FIGS. 4 and 5. A first organic semiconductor transport layer36 or 126 is operatively associated with the emitter electrode forinjecting charge carriers having a first charge type into thesemiconductor layer. The first organic transport layer is positionedbetween the emitter electrode and the semiconductor layer and forms partor all of a first structure positioned between the emitter electrode andthe inorganic semiconductor layer that facilitates diffusion of thecharge carriers having the first charge type into the inorganicsemiconductor layer and suppresses diffusion of charge carriers having asecond charge type opposite from the first charge type. In someembodiments, the organic transport layer suppresses diffusion of thecharge carriers having the second charge type from the inorganicsemiconductor layer towards the emitter electrode in addition tofacilitating diffusion of the charge carriers having the first chargetype into the inorganic semiconductor layer. A second transport layer 86is operatively associated with the base electrode 84 for transportingcharge carriers having a second charge type opposite from the firstcharge type. In other words, if the first organic semiconductortransport layer comprises hole transport material for injecting holesinto the crystalline inorganic semiconductor layer, the second transportlayer is selected for transporting electrons. The second transport layeris positioned between the base electrode and the semiconductor layer andforms all or part of a second structure operatively associated with thebase electrode for transporting charge carriers having the second chargetype and suppressing diffusion of charge carriers having the firstcharge type. The second structure is positioned between the baseelectrode and the inorganic semiconductor layer. A third transport layer76 or 136 is operatively associated with the collector electrode fortransporting charge carriers having the first charge type and ispositioned between the collector electrode and the inorganicsemiconductor layer. Carrier blocking layers such as layers 38, 88, 78are provided within emitter, base and collector contact structures inone or more embodiments. Carrier blocking may be provided by discretelayers or by carrier transport and/or passivation layers. A passivationlayer 40 or 116 is provided in one or more embodiments. The passivationlayer saturates dangling bonds at the surface of an inorganiccrystalline semiconductor substrate and reduces minority carrierrecombination at the surface of the substrate. In one or moreembodiments, the passivation layer suppresses majority carrier diffusionfrom the inorganic semiconductor substrate into the organic contactstructure and therefore functions as a carrier blocking layer inaddition to a surface passivation layer. The organic semiconductormaterial includes pentacene in one or more embodiments. The inorganiccrystalline semiconductor layer is an n-type crystalline silicon layerin some embodiments and a p-type crystalline silicon layer in otherexemplary embodiments. A silicon substrate is recessed in someembodiments, such as shown in FIG. 5, wherein the emitter structure andcollector structure are only partially formed over the inorganicsemiconductor layer.

An exemplary method includes obtaining a bipolar junction transistorincluding a crystalline inorganic semiconductor layer and emitter, baseand collector contact structures (for example, structures 30, 80, 70 asshown in FIG. 4) operatively associated with the crystalline inorganicsemiconductor layer (for example, layer 102 in FIG. 4). The emittercontact structure comprises an organic semiconductor material andincludes one of electron-transport/hole blocking andhole-transport/electron-blocking layers. The contact structure 50 shownin FIG. 2B is an exemplary electron-transport/hole blocking structure.The contact structure 30 shown in FIGS. 2A and 4 is an exemplaryhole-transport/electron blocking structure. Further steps includecausing diffusion of a first type of charge carrier (e.g. electrons orholes) into the crystalline inorganic semiconductor layer from theemitter contact structure and, within the emitter contact structure,suppressing diffusion of a second type of charge carrier opposite to thefirst type of charge carrier from the crystalline inorganicsemiconductor layer. The method further includes causing the bipolarjunction transistor to provide an electrical current (e.g. a DC current)to an electronic device such as a passive device. In one or moreembodiments, the passive device is an OLED 28 as shown in the circuitdiagram provided in FIG. 1.

A structure provided in accordance with one or more embodiments includesa crystalline inorganic semiconductor layer adjoining an electricallyinsulating layer and an array of bipolar junction transistors. Each ofthe bipolar junction transistors includes an emitter contact structureoperatively associated with the crystalline inorganic semiconductorlayer. The emitter contact structures each comprise one of anelectron-transport and hole blocking structure and a hole-transport andelectron-blocking structure for diffusing a first type of charge carrierinto the crystalline inorganic semiconductor layer and suppressing thediffusion of a second type of charge carrier from the crystallineinorganic semiconductor layer. FIG. 2B shows a contact structure 50 thatprovides electron transport and hole blocking FIG. 2A shows an emittercontact structure 30 that provides hole transport and electron blocking.In both exemplary structures 30, 50, the transport layer is an organicsemiconductor material layer. As discussed above with respect to FIG. 3,pentacene is an organic semiconductor layer that can be employed forhole transport. An array of thin film switching transistors iselectrically connected to the bipolar junction transistors. FIG. 1illustrates exemplary electrical connections of switching transistorswith driver thin film transistors such as the bipolar junction devices100, 110 as described above. FIGS. 8 and 9 provide more detailed viewsof electrical connections between switching transistors 204, 304 andbipolar junction transistors 202. An array of passive devices is furtherprovided, each of the passive devices being electrically connected toone of the bipolar junction transistors. FIG. 1 shows connections ofpassive devices, namely OLEDs 28, to bipolar junction transistors 26having structures as described above with respect to FIGS. 4 and 5. Insome embodiments, each emitter contact structure includes an emitterelectrode 34 comprised of high workfunction material such as gold and anorganic semiconductor hole transport layer 36 for injecting holes intothe crystalline inorganic semiconductor layer by diffusion. In otherexemplary embodiments, each emitter contact structure includes anemitter electrode 54 comprised of low workfunction material and anelectron transport layer 56 for injecting electrons into the crystallineinorganic semiconductor layer by diffusion.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. Terms such as “above”, “below”, “top”and “bottom” are generally employed to indicate relative positions asopposed to relative elevations unless otherwise indicated. It shouldalso be noted that, in some alternative implementations, the steps ofthe exemplary methods may occur out of the order noted in the figures.For example, two steps shown in succession may, in fact, be executedsubstantially concurrently, or certain steps may sometimes be executedin the reverse order, depending upon the functionality involved.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A bipolar junction transistor comprising: a dopedinorganic semiconductor layer; an emitter electrode; a base electrode; acollector electrode; a first structure operatively associated with theemitter electrode for transporting charge carriers having a first chargetype into the inorganic semiconductor layer and suppressing diffusion ofcharge carriers having a second charge type opposite from the firstcharge type from the inorganic semiconductor layer, the first structurebeing positioned between the emitter electrode and the inorganicsemiconductor layer and including a first organic semiconductor carriertransport layer for transporting the charge carriers having the firsttype; a second structure operatively associated with the base electrodefor transporting charge carriers having the second charge type into theinorganic semiconductor layer and suppressing diffusion of chargecarriers having the first charge type from the inorganic semiconductorlayer, the second structure being positioned between the base electrodeand the inorganic semiconductor layer, and a third structure operativelyassociated with the collector electrode for transporting charge carriershaving the first charge type into the inorganic semiconductor layer andsuppressing diffusion of charge carriers having the second charge typefrom the inorganic semiconductor layer, the third structure beingpositioned between the collector electrode and the inorganicsemiconductor layer.
 2. The bipolar junction transistor of claim 1,wherein each of the second and third structures comprises an organicsemiconductor layer for transporting charge carriers.
 3. The bipolarjunction transistor of claim 1, wherein the inorganic semiconductorlayer comprises an n-type crystalline silicon layer, the charge carriershaving the first charge type are holes and the charge carriers havingthe second charge type are electrons.
 4. The bipolar junction transistorof claim 3, wherein the emitter electrode includes a layer of highworkfunction material.
 5. The bipolar junction transistor of claim 1,further including a passivation layer between the semiconductor layerand the first, second and third structures, the passivation layerdirectly contacting the inorganic semiconductor layer.
 6. The bipolarjunction transistor of claim 5, wherein at least one of the passivationlayer and the first organic semiconductor charge transport layercomprises a blocking layer for suppressing diffusion of charge carriershaving the second type.
 7. The bipolar junction transistor of claim 1,wherein the inorganic semiconductor layer comprises a p-type crystallinesilicon layer, the charge carriers having the first charge type areelectrons and the charge carriers having the second charge type areholes.
 8. The bipolar junction transistor of claim 1, wherein the secondand third structures include charge transport layers comprised oforganic semiconductor material.
 9. The bipolar junction transistor ofclaim 1, wherein the first structure further including a blocking layerfor suppressing diffusion of charge carriers having the second type. 10.The bipolar junction transistor of claim 9, wherein the first structurefurther includes a passivation layer directly contacting thesemiconductor layer.
 11. The bipolar junction transistor of claim 10,wherein at least one of the passivation layer and the first organicsemiconductor carrier transport layer comprises the blocking layer. 12.The bipolar junction transistor of claim 11, wherein the passivationlayer includes a layer of organic material directly contacting theinorganic semiconductor layer and the second and third structurescomprise organic materials.